In a known power distribution system, current is distributed to different device branches or electrical devices with the aid of switches (circuit breakers) in a switch mechanism, especially low-voltage circuit breakers. These switches are each designed for a given rated current, and cut off the flow of current when a fault (such as a short circuit) occurs. Only those device branches which are affected by the fault or are closest to the fault are cut off. Such an operation is called selective breaking.
Inside each switch are provided a current transformer and a trip unit. The current transformer detects current flowing through the switch device, while the trip unit checks whether this current meets a specified condition (such as a current condition).
When selective breaking is implemented, these switches communicate with each other. When a situation occurs in which the specified current condition is met due to a short circuit, a switch located downstream in the power supply direction notifies a switch lying upstream of itself of this situation by means of a signal (e.g. a locking signal or delay signal). In this case, this upstream switch, which has similarly discovered the short circuit, temporarily refrains from tripping, instead waiting for a given delay time to observe whether the downstream switch trips. If the downstream switch has still not tripped when the delay time expires, then the upstream switch cuts off the current itself. Such a selective breaking solution is generally referred to as Zone Selective Interlocking (or ZSI for short).
Furthermore, in some power distribution systems, the power supply direction may change. For instance, if there are multiple feeder power supplies, the disconnection or connection of one power supply might cause a reversal of the direction of flow of current through one or more switches. This will cause a change in the upstream/downstream relationship amongst some of the switches in the ZSI system, so that the directions in which ZSI signals (locking signals or delay signals) are transmitted must be adjusted appropriately in order to achieve selective breaking.
Siemens has proposed a corresponding solution in which a technically simple communicative connection amongst switches, which is able to adapt to changes in the power supply direction, is realized. For convenience of description, this text refers to this type of zone selective interlocking, in which changes in power supply direction are taken into account, as directional zone selective interlocking (i.e. Directional ZSI, or DZSI for short). However, existing DZSI solutions do not take into account inspection of faults in the DZSI communication links. If a DZSI communication link fails (i.e. develops a fault) and this cannot be discovered promptly, then serious loss may result.
Furthermore, a DZSI system may comprise different types of switches. FIGS. 1 and 2 each show a typical mixed DZSI system formed by a 3WL ACB DZSI subsystem and an SnG MCCB DZSI subsystem. 3WL ACB and SnG MCCB are two typical switch types from Siemens, wherein the 3WL ACB is a frame-type circuit breaker, while the SnG MCCB is a low-voltage molded-case circuit breaker. The ZSI devices of SnG MCCBs currently on the market all lack directional selection functionality, generating and transmitting ZSI signals in accordance with old protocols and port definitions, so such a mixed DZSI system has higher requirements in terms of compatibility of inter-switch communication. A major difficulty currently faced is how to enable the ZSI device of an SnG MCCB to transmit DZSI signals (ZSI signals transmitted in a DZSI system may be referred to as DZSI signals) reliably and in an orderly way, and how to realize automatic inspection of DZSI communication link faults between switches of different types.